34 MASS. EXPERIMENT STATION BULLETIN 351 



Zinc Sulfate 



As a supplement to seed treatment with red copper oxide, which does not 

 wholly prevent post -germination damping-off, Horsfall (48) successfully used 

 zinc oxide 0.5 to 1.0 ounce per square foot, applying it to the surface of the soil 

 after seeding. He did not find zinc sulfate safe although germination of tomato 

 was in some cases improved and damping-off was at least partly controlled 

 by 10 gm. 



The sulfate is, of course, much more soluble and might, in the absence of seed 

 treatment, be expected to give more protection than the oxide. Zinc sulfate was 

 accordingly used in these experiments with the thought that it may be less harm- 

 ful to some species than to tomato and useful at least with them. It was always 

 applied to soil before seeding and in solution. 



Post-emergence damping-off, caused principally by Pythium, partly by Rhi- 

 zoctonia, was not completely controlled by less than 16 gm. , but there was usually 

 some control of pre-emergence damping-off, with germination improved accord- 

 ingly, by 12 gm. or more. Zinc sulfate, however, in quantities effective against 

 damping-off, interfered somewhat with the growth of most species used, as shown 

 by the following tabulation. 



Sweet William and sweet pea were the only species which were at all satisfac- 

 torily protected against damping-off by zinc sulfate without some chemical injury. 



Sulfuric Acid 



Sulfuric acid is known to have some value as a soil disinfectant and it has been 

 used by other investigators (43, 88, 42) for the control of damping-off of conifers. 

 Recommended rates of application per square foot vary considerably, from about 

 3 to about 14 gm., diluted with water and applied to soil at the time of seeding. 

 Even these quantities have been found to be unsafe in some soils and with some 

 species, and as little as 5.1 gm. was observed by Toumey and Li (88) to retard 

 the growth of seedlings of white pine, spruce, and hemlock. Not much use seems 

 to have been made of it with herbaceous plants. 



The writer diluted sulfuric acid (95 percent, sp. gr. 1.84) so as to give the desired 

 number of grams of this acid per square foot when 2 quarts of the solution were 

 applied. Seeds were sown within 24 hours after soil treatment. Soils used had 

 initial pH values of 6.4 to 6.6. Soil pH values, 8 weeks after treatment in a 



